Adhesives are generally classified into a solvent-type adhesive and an aqueous emulsion-type adhesive. In these adhesives, the solvent-type adhesive has the disadvantages of causing inflammation and fire, the hygienic problem, and environmental pollution, each caused by the solvent used. On the other hand, the aqueous emulsion-type adhesive does not have such disadvantages.
An aqueous emulsion-type adhesive is usually prepared by dispersing the fine particles of a water-insoluble resin having particle sizes of from about 0.05 to 10 .mu.m in water as a medium. Accordingly, for improving the dispersibility of the fine particles of the resin in water, it is necessary that a hydrophilic component exists at the surface layer portions of the fine particles.
However, when such an aqueous emulsion-type adhesive is used, the hydrophilic component remains in the adhesive layer or film formed after adhesion and the layer containing such a hydrophilic component is poor in water resistance.
The following methods are known as methods for improving the water resistance of such an aqueous emulsion-type adhesive.
JP-A-56-93702 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of using acetoacetylated polyvinyl alcohol as an emulsifier in the emulsion-polymerization of a vinyl compound in an aqueous medium. Further, JP-A-57-202364, JP-A-57-59971, and JP-A-58-24435 disclose a method of using an adhesive comprising an aqueous emulsion containing acetoacetylated polyvinyl alcohol as a protective colloid. However, in these techniques, the effect of improving the water resistance is insufficient.
A synthetic resin emulsion containing acetoacetylated polyvinyl alcohol as a protective colloid has a problem that the storage stability is poor and hence the emulsion (adhesive) is coagulated with the passage of time. JP-A-58-201837 discloses adding a hydrogensulfite to an aqueous emulsion-type adhesive to improve the storage stability of the emulsion but since an unpleasant odor (the odor of sulfur series compounds) due to the decomposition products of the hydrogensulfite occurs, there is inevitably a severe restriction for the use of such an adhesive.
JP-A-58-217571, JP-A-59-1581, JP-A-63-142079, JP-A-2-155975, JP-A-59-217780, and JP-A-59-161474 disclose a method of increasing the water resistance of an adhesive comprising an aqueous emulsion containing acetoacetylated polyvinyl alcohol as a protective colloid by mixing the aqueous emulsion with various kinds of modifiers such as an isocyanate series compound, an epoxy compound, a compound having a radical-forming ability, an oxidizing agent, etc.
However, since such a method has a short pot life (workable time), there is a problem that the restriction on the practical use is large.
As the techniques for covering the above disadvantage that the pot life of the mixtures is short, JP-A-60-202176, JP-A-61-21171, JP-A-61-78883, and JP-A-60-240777 attempt to improve the pot life by using a two part type adhesive wherein the main component is coated on one surface of a material and the curing agent is coated on one surface of another material, and both the materials are bonded, causing a curing reaction (a honeymoon-type adhesive), or the main component and the curing agent are mixed immediately before use.
However, even the above techniques are, as a matter of course, restricted to the use under a specific condition and are unsuitable for the use of widely usable adhesives.
As a conventional technique relating to a synthetic resin emulsion prepared by copolymerizing an acetoacetyl group-containing copolymerizable monomer, a homopolymer of an emulsion-polymerizable unsaturated keto compound or a copolymer of the emulsion-polymerizable unsaturated keto compound and other copolymerizable monomer is used as described in German Patent Publication 1,495,704. JP-A-52-22030 also discloses use of a copolymer emulsion comprising an olefinically unsaturated monomer and a polymerizable acetoacetic acid ester as a coating material.
Examples of a combination of monomers are many combinations such as the combination of vinyl acetate and an acetoacetic acid allyl ester, etc., and in these examples, there is a combination of ethylene, vinyl acetate, and an acetoacetic acid allyl ester.
However, in JP-A-52-22030 described above, there are no descriptions of anticipating that in the case of using an ethylene-vinyl ester copolymer emulsion obtained by copolymerizing an acetoacetyl group-containing monomer having the specific composition as in the present invention, a high water resistance is obtained.
Also, JP-A-52-22085, JP-A-52-22086, and JP-A-53-2588 show use of a synthetic resin emulsion obtained by copolymerizing a polmerizable acetoacetic acid ester (e.g., acetoacetic acid allyl ester) in a polymer system in which the composition of various kinds of monomers are specified, as mainly a coating material. Furthermore, JP-A-56-76148 discloses a copolymer emulsion of an olefinically unsaturated compound and a polymerizable diacetylacetic acid ester (e.g., diacetylacetic acid allyl ester). Also, JP-B-3-66322 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a copolymer of an acetylacetoxyalkyl allyl ether [e.g., (2-acetylacetoxy-propyl) allyl ether] and other copolymerizable monomer.
However, in any of the above-described patent publications, there is no disclosure on an ethyl-vinyl ester copolymer emulsion of a specific composition obtained by copolymerizing a specific amount of an acetoacetyl group-containing polymerizable monomer, and in particular, there is no disclosure on that the copolymer emulsion has a specific toluene-insoluble content and also the copolymer emulsion using a specific amount of polyvinyl alcohol as a protective colloid is preferred. Also, there is no description on use of the copolymer emulsion as adhesives requiring a high water resistance.